1. Requirements Become the E-R Data Model
After the requirements have been gathered, they are transformed into an Entity Relationship (E-R) Data Model.
The most important elements of E-R Models are:
Entities
Attributes
Identifiers
Relationships
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2. Entity Class versus Entity Instance
An entity class is a description of the structure and format of the occurrences of the entity.
An entity instance is a specific occurrence of an entity within an entity class.
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3. Entity Class and Entity Instance
Figure 4-2: The ITEM Entity and Two Entity Instances
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4. Attributes
Entities have attributes that describe the entity’s characteristics:
ProjectName
StartDate
ProjectType
ProjectDescription
Attributes have a data type and properties.
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5. Identifiers Entity instances have identifiers.
An identifier will identify a particular instance in the entity class:
SocialSecurityNumber
StudentID
EmployeeID
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6. Identifier Types Uniqueness Composite
Identifiers may be unique or nonunique.
If the identifier is unique, the data value for the identifier must be unique for all instances.
Composite
A composite identifier consists of two or more attributes.
E.g., OrderNumber & LineItemNumber are both required.
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7. Levels of Entity Attribute Display
Figure 4-3: Levels of Entity Attribute Display
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8. Relationships
Entities can be associated with one another in relationships.
Relationship degree defines the number of entity classes participating in the relationship:
Degree 2 is a binary relationship.
Degree 3 is a ternary relationship.
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9. Degree 2 Relationship: Binary
Figure 4-4: Example Relationships
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10. Degree 3 Relationship: Ternary
Figure 4-4: Example Relationships
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11. One-to-One Binary Relationship
A single entity instance in one entity class is related to a single entity instance in another entity class.
An employee may have no more than one locker; and
A locker may only be accessible by one employee
(a) One-to-One Relationship
Figure 4-5: Three Types of Binary Relationships
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12. One-to-Many Binary Relationship
1:N (one-to-many)
A single entity instance in one entity class is related to many entity instances in another entity class.
A quotation is associated with only one item; and
An item may have several quotations
(b) One-to-Many Relationship
Figure 4-4: Three Types of Binary Relationships
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13. Many-to-Many Binary Relationship
N:M (many-to-many)
Many entity instances in one entity class is related to many entity instances in another entity class:
a supplier may supply several items; and
a particular item may be supplied by several suppliers.
(c) Many-to-Many Relationship
Figure 4-5: Three Types of Binary Relationships
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14. Maximum Cardinality
Relationships are named and classified by their cardinality, which is a word that means count.
Each of the three types of binary relationships shown above have different maximum cardinalities.
Maximum cardinality is the maximum number of entity instances that may participate in a relationship instance—one, many, or some other fixed number.
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15. Minimum Cardinality
Minimum cardinality is the minimum number of entity instances that must participate in a relationship instance.
These values typically assume a value of zero (optional) or one (mandatory).
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16. Cardinality Example
Maximum cardinality is many for both ITEM and SUPPLIER.
Minimum cardinality is zero (optional) for ITEM and one (mandatory) SUPPLIER.
A SUPPLIER does not have to supply an ITEM.
An ITEM must have a SUPPLIER.
Figure 4-6: A Relationship with Minimum Cardinalities
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17. Entity-Relationship Diagrams
The diagrams in previous slides are called entity-relationship diagrams.
Entity classes are shown by rectangles.
Relationships are shown by diamonds.
The maximum cardinality of the relationship is shown inside the diamond.
The minimum cardinality is shown by the oval or hash mark next to the entity.
The name of the entity is shown inside the rectangle.
The name of the relationship is shown near the diamond.
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18. HAS-A Relationships
The relationships in the previous slides are called HAS-A relationships.
The term is used because each entity instance has a relationship to a second entity instance:
An employee has a badge.
A badge has an employee.
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19. Types of Entity-Relationship Diagrams
Information Engineering (IE) [James Martin 1990]—Uses “crow’s feet” to show the many sides of a relationship, and it is sometimes called the crow’s foot model.
Integrated Definition 1, Extended 3 (IDEF1X) is a version of the E-R model that is a national standard.
Unified Modeling Language (UML) is a set of structures and techniques for modeling and designing object-oriented programs (OOP) and applications
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20. Crow’s Foot Example: One-to-Many Relationship
Figure 4-7: Two Versions of a 1:N Relationship
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21. Crow’s Foot Symbols Figure 4-8: Crow’s Foot Notation
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22. Crow’s Foot Example: Many-to-Many Relationship
Figure 4-9: Two Versions of an N:M Relationship
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23. Weak Entity
A weak entity is an entity that cannot exist in the database without the existence of another entity.
Any entity that is not a weak entity is called a strong entity.
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24. ID-Dependent Weak Entities
An ID-Dependent weak entity is a weak entity that cannot exist without its parent entity.
An ID-dependent weak entity has a composite identifier.
The first part of the identifier is the identifier for the strong entity.
The second part of the identifier is the identifier for the weak entity itself.
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25. ID-Dependent Weak Entity Examples
Figure 4-10: Example ID-Dependent Entities
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26. Weak Entity Relationships
The relationship between a strong and weak entity is termed an identifying relationship if the weak entity is ID-dependent.
Represented by a solid line
The relationship between a strong and weak entity is termed a nonidentifying relationship if the weak entity is non-ID-dependent.
Represented by a dashed line
Also used between strong entities
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27. Weak Entity Identifier: Non-ID-dependent
All ID-dependent entities are weak entities, but there are other entities that are weak but not ID-dependent.
A non-ID-dependent weak entity may have a single or composite identifier, but the identifier of the parent entity will be a foreign key.
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28. Non-ID-Dependent Weak Entity Examples
Figure 4-11: Weak Entity Examples
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29. Strong and Weak Entity Examples
Figure 4-12: Examples of Required Entities
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30. Associative Entities
An associative entity (also called an association entity) is used when there are attributes that are associated with the relationship between two entities rather than with either of the two entities themselves.
A new entity is then created to:
Link the two original entities
Hold the attributes
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31. Associative Entities (Cont’d)
Figure 4-13: The Associative Entity
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32. Subtype Entities
A subtype entity is a special case of another entity called supertype.
An attribute of the supertype may be included that indicates which of the subtypes is appropriate for a given instance; this attribute is called a discriminator.
Subtypes can be exclusive or inclusive.
If exclusive, the supertype relates to at most one subtype.
If inclusive, the supertype can relate to one or more subtypes.
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33. Subtype Entity Identifiers
The relationships that connect supertypes and subtypes are called IS-A relationships because a subtype is the same entity as the supertype.
The identifier of a supertype and all of its subtypes is the same attribute.
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34. Subtype Entity Examples
Figure 4-14: Example Subtype Entities
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35. Recursive Relationships
It is possible for an entity to have a relationship to itself—this is called a recursive relationship.
Figure 4-15: Example Recursive Relationship
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